The output voltage of the battery is often used as an indicator of the energy remaining in the battery. That output voltage may be monitored by a voltage monitoring circuit, either internally connected to the device to which the battery supplies energy or externally connected to that device. The output voltage of some types of batteries, such as alkaline manganese dioxide Zn/MnO.sub.2 batteries (hereinafter referred to as "alkaline" batteries) gradually decreases, as shown in FIG. 1A (corresponding to FIG. 7.5 of the Handbook of Batteries, edited by David Linden, 1984). When the monitoring circuit has detected that the output voltage has decreased below a predetermined voltage level, there might be enough energy left in the battery for the device to complete a critical ongoing task, such as drug delivery in an iontophoretic drug delivery system, as described below, or to perform an essential power-down function, such as memory backup in a battery-powered computer. These types of batteries, however, may not be preferred for certain applications which require batteries with high energy and high current capacity.
Other conventional batteries, such as zinc/silver oxide batteries (Zn/Ag.sub.2 O, hereinafter referred to a "silver oxide batteries"), are characterized by a substantially flat output voltage over time, until the cells of the battery die, at which time the output voltage sharply decreases, as shown by FIG. 1B (corresponding to FIG. 9.4 of the Handbook of Batteries). Despite this discharge characteristic, silver oxide batteries are preferred for certain electrical applications because they are small, thin and light, and deliver a high amount of current for a long period of time. When the voltage monitoring circuit has detected that the output voltage of the battery has begun to decrease sharply, there may not be enough energy left in the battery, however, for the device to complete a critical ongoing task or to perform an essential power-down function. Therefore, there is a need for a high quality, high-current delivering battery, such as the conventional silver oxide battery, which also has enough remaining energy, after the voltage monitoring circuit has detected the sharp decrease in the output voltage, to allow the device to complete its task or to power-down.
Section 8.5.7 of the Handbook of Batteries describes a "stepped-voltage" battery which produces a well-defined step in the output voltage prior to its complete discharge, as shown in FIG. 1C (corresponding to FIG. 8.25 of the Handbook of Batteries). This voltage step occurs well before the end of battery life so that, after the voltage monitoring circuit has detected the voltage step, enough energy remains in the battery to allow the device to complete a final task or to power-down. The stepped-voltage battery is made by using materials in the cathode or the anode of the battery which discharge at a different potential from the base electrode.
In particular, FIG. 1C shows a nine-cell battery having a stepped battery voltage discharge curve V.sub.total, which is produced by serially connecting seven zinc/mercuric oxide cells that together are characterized by the substantially flat voltage discharge curve V.sub.2, and two hybrid cells that together are characterized by the stepped voltage discharge curve V.sub.1. The hybrid cells have cathodes in which part of the mercuric oxide has been replaced by cadmium oxide in a sufficient quantity to leave each hybrid cell with the same balanced capacity. When all of the mercuric oxide has been reduced in the hybrid cells, that is, the hybrid cells have discharged, their combined voltage falls by 1.5 Volts (750 millivolt per hybrid cell), as shown in curve V.sub.1. This causes the combined voltage V.sub.total to decrease by 1.5 Volts. This sudden, large drop in the output voltage can be easily detected by the voltage monitoring circuit, and thus can serve to trigger an alarm indicating the need for battery replacement, or to warn the device that any ongoing task should be a final task or that the device should begin powering down. The size of the voltage step can be adjusted, for example, by increasing or decreasing the number of hybrid cells in the battery. Further, during manufacture of the stepped-voltage battery, the voltage step can be arranged to occur at varying points during the life of the battery. For example, in the nine-cell battery of FIG. 1C, the voltage step was arranged to occur at about 60% (650 hours) of the overall life (1100 hours).
The above-described stepped-voltage batteries are, however, limited in their use, especially as a substitute for silver oxide batteries and the like. First, all the cells of the stepped-voltage battery are arranged in a relatively large, wide and heavy package, making its use impractical for small or thin electronic devices. Second, relative to silver oxide batteries, stepped-voltage batteries are expensive and have a lower current capacity. Third, to meet all of the different energy requirements of various devices, a device manufacturer would need to order and stock, unfortunately, many different types of stepped-voltage batteries. Finally, although the time at which the voltage step of the stepped-voltage battery occurs can be set as described above, that setting is set during manufacturing and cannot be adjusted thereafter. It would be more desirable to be able to use a battery with which the time of the voltage step can be adjusted while the device is being operated. Such time adjustment can be based on the operating conditions of the device using, for example, computer control.
Thus, despite the availability of stepped-voltage batteries, there is still a need for a time-adjustable, stepped-voltage output when use of a more practical and desirable conventional battery, such as a silver oxide battery, is required by the device.